The present invention relates generally to fuel cell systems and more particularly to fuel cell systems with controlled anode exhaust.
Fuel cell systems currently being developed for power generation applications operate on basically the same electrochemical principles as a battery system to generate electricity. Fuel cell systems generally consist of a reformer, a fuel cell stack, and an power converter (generator).
In a typical power generation system, first the reformer, or fuel processor, converts a feedstock, typically a hydrocarbon fuel such as gasoline or natural gas, into a hydrogen rich gas. Many types of reformers can be used to convert the feedstock, including steam reformers, auto thermal reformers, partial oxidation reformers, preferential oxidation reformers, and high and low temperature shift processing reformers.
Next, the hydrogen gas stream is removed from the reformer and fed into an anode chamber of a fuel cell stack for electrochemical oxidation. In this step, the hydrogen rich gas is oxidized in the presence of a catalyst contained on the walls of the anode chamber to form hydrogen protons and electrons. An electrolytic membrane allows the hydrogen protons to enter a cathode chamber, where the hydrogen protons react with oxygen from air to form water and heat that is released from the cathode chamber into the atmosphere. The electrons within the cathode chamber exit the fuel cell along an external remote load wire and are converted by the generator to usable power. By stacking a few hundred fuel cells together to form a fuel cell stack, the amount of power generated can be substantially increased. Unoxidized fuel within the anode chamber exits the fuel cell and enters a combuster for burning.
During normal operating conditions, wherein the electrical load demand on the fuel cell remains relatively steady, the oxidation reaction efficiency of the fuel cell is relatively constant at approximately 80-90%. The amount of unoxidized fuel thus entering the combuster therefore remains relatively constant, and as such the combuster temperature that is a function of the amount of unoxidized fuel burned within the combuster is maintained within an acceptable range.
However, under transient operating conditions, wherein the electrical load demand on the fuel cell could be decreased, the amount of unoxidized fuel within the anode chamber substantially increases. Burning of this excess unoxidized fuel in the combuster leads to the generation of high temperatures and accelerated corrosion (or melting) of the combuster material. Such temperature transients in the combuster require expensive construction materials and higher quality insulation to provide thermal stability in the combuster. Combustion of the unoxidized fuel in the combuster also reduces the overall fuel system efficiency.
Another alternative is to recycle the unoxidized fuel through the inlet anode gas stream of the fuel cell. However, recycling and injection of hydrogen depleted tail gas in the anode gas stream leads to increased parasitic power loss.
It is thus highly desirable to control the temperature in the combuster by improving the electrochemical oxidation of the fuel within the fuel cell during transient load conditions.
The present invention overcomes the above identified deficiencies of presently available fuel cell systems by providing a means for oxidizing heated reformed fuel gas in a fuel cell during transient load conditions. Electrochemical oxidation of the fuel in the fuel cell minimizes or prevents a buildup of anode gas in the anode chamber. Also, combustion of unoxidized gas is minimized in the combuster, thereby preventing excess heat buildup in the combuster that could damage the fuel processing assembly.
The above objects are accomplished by providing a methodology for operating a fuel cell power generation system in conjunction with an energy storage system in a parallel mode operation to prevent transient increases in the combustion of anode gas during changes in electrical load demand. The energy storage device consists of either a battery pack or a bank of capacitors/super capacitors that are attached in parallel to the fuel cell system such that both can provide power during normal and/or transient operations. Normally unoxidized reformed gas within the anode chamber during transient conditions is oxidized and the electrical energy created by the oxidation stored in an energy storage device. This prevents the discharge of larger than normal amounts of reformer gas to the combuster, gas that normally results in very high temperature combustion within the combuster (approximately 1400-1500 degrees Celsius) that can cause corrosion (or melting) of combuster material or otherwise damage to the fuel processing assembly.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.